The present invention relates to a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus, and more particularly to a method of stabilizing a magnetic field of a magnetic field generating apparatus employing permanent magnets, a magnetic field generating apparatus that generates a stable magnetic field, and a magnetic resonance imaging apparatus that employs such a magnetic field generating apparatus.
A magnetic field generating apparatus that employs permanent magnets for magnetic resonance imaging has a pair of permanent magnets facing each other in the vertical direction with a space interposed, and yokes constituting a magnetic circuit for the permanent magnets. In order to generate a stable magnetic field while preventing a change in magnetic field strength due to a variation in the ambient temperature, the magnetic field generating apparatus is kept at a constant temperature. The constant temperature is selected so that it is slightly higher than the ordinary temperature, and yet the permanent magnet does not lose its magnetism. For example, a temperature of 30.degree. C. is selected.
The temperature is maintained by using an electric heater as a heat supply source, and the heat release amount of the electric heater is controlled by a controller. The controller is supplied with a detected signal indicative of the temperature measured at a predetermined position in the magnetic field generating apparatus. The temperature measurement point to obtain the detected signal is positioned within a yoke immediately above an upper permanent magnet, for example. The controller controls the heat release amount of the electric heater so that a value of the detected signal is equalized to a predetermined value.
The electric heater employed is one having a large heat release amount corresponding to a large heat capacity of the magnetic field generating apparatus, and the controller on-off controls the electric heater with PWM (pulse-width-modulation) signals. The duty ratio between ON and OFF is PID-controlled (proportional-plus-integral-plus-derivative-controlled) according to the deviation of the detected signals from the predetermined value.
In attempting to maintain the constant temperature of the magnetic field generating apparatus by the temperature control as described above, a difference arises between the temperatures of the upper and lower permanent magnets when a temperature difference is increased between upper and lower positions in a space in which the magnetic field generating apparatus is installed, as is often the case with a mobile magnetic resonance imaging apparatus in the hard winter, etc., leading to a problem that the stability of the magnetic field is deteriorated.
Moreover, since an electric heater having a large heat release amount is employed, frequency of switching between ON and OFF is increased when the temperature control is conducted with a small temperature deviation, i.e., in the low-ripple state, causing the control to be prone to instability due to a lag of heat conduction within the magnetic field generating apparatus. Furthermore, the increase in frequency of switching between ON and OFF gives rise to noises, which deteriorates the quality of captured images.